Abstract: We argue that the rise of regionalism reflects a world polity theory (WPT) logic, whereby it is legitimated and promoted through amendments to the "script of statehood" by credentialed experts, ritually and isomorphically taken up by states as a function of their need for legitimacy, and enacted and reproduced notwithstanding profound decoupling between embraced and realized goals. We test and find support for these claims on the empirical terrain defined by regional trade agreements (RTAs), using a variety of data and methods. While political economy models have much to tell us about the rise of regionalism, the sociological dynamics of this and related processes of institutional change deserve heightened attention in an increasingly integrated world polity.